The present invention generally relates to a power circuit for converting a supply voltage and outputting the voltage converted, and more particularly relates to a technique of limiting an inrush current during the initial operation of the power circuit.
An integrated circuit may include several components operating at a voltage different from a given supply voltage. In such a situation, a power circuit is used to convert the supply voltage into a required voltage and output the voltage converted.
FIG. 17 illustrates a configuration of a conventional power circuit. The power circuit shown in FIG. 17 includes an output transistor 1, a differential amplifier 2 and a smoothing capacitor 3. The output transistor 1 supplies a current to a load connected to an output terminal 4. The differential amplifier 2 controls the current supplied by the output transistor 1 at such a value as equalizing an output voltage OUT output through the output terminal 4 with a reference voltage REF applied to a reference terminal 5. And the smoothing capacitor 3 is provided to remove abrupt fluctuations in the output current of the load.
In the conventional power circuit shown in FIG. 17, however, the output transistor 1 can supply a maximum possible current during the initial operation of the circuit. Thus, an excessively large current, i.e., a so-called "inrush current", might flow from a power supply 100 into the smoothing capacitor 3 via the output transistor 1. Such an inrush current might adversely affect, or damage, the devices of an LSI including the power circuit or external components.
According to an exemplary technique of suppressing the generation of inrush current, an inrush-current-limiting bipolar transistor is connected in series to an output transistor (see Japanese Laid-Open Publication No. 8-154338, for example). In general, the ratio of the emitter-collector current of a bipolar transistor to the base current thereof is equal to or smaller than the current amplification ratio of the base current. Thus, the proposed circuit allegedly suppresses the generation of the inrush current by limiting the base current of the inrush-current-limiting bipolar transistor.
In such a case, however, a process for fabricating the bipolar transistor is required. Accordingly, if the output transistor and the differential amplifier are implemented as MOS transistors, for example, then the fabrication process of such a circuit gets overly complicated.
Also, the inrush-current-limiting transistor is connected in series to the output transistor 1. Accordingly, the channel of the inrush-current-limiting transistor should be wide enough to supply a large current as the output current, thus increasing the layout area and the overall cost of the power circuit. In addition, when a large current is output, a voltage drop between the emitter and collector of the inrush-current-limiting transistor is large. Thus, it is impossible to supply an output voltage approximating the supply voltage. To suppress the voltage drop caused by the inrush-current-limiting transistor, the channel width thereof should be increased. However, the layout area of the power circuit further increases in such a case.